Thermal constraints on crustal rare gas release and migration: Evidence from Alpine fluid inclusions

Abstract

The Palfris marl of the Helvetic Alpine Nappes contains four distinct vein fill generations. CH 4-rich gas is found in abundant fluid inclusions within these carbonate veins, while free CH 4 gas has also been produced from exploratory boreholes through this formation. The stable isotope and helium, neon, and argon isotopic composition of these fluids has been determined. A constant radiogenic 40Ar concentration of 1.25 ± 0.13 (1σ) ppm in these differently sited fluids requires an intimate association between the 40Ar rad source and the hydrocarbon phase. This can only be reasonably explained if the 40Ar rad was input into the hydrocarbon phase during hydrocarbon generation, migration, or storage prior to entrapment in the fluid inclusions. Stable isotope results constrain the maturity of hydrocarbon production, while fluid inclusion formation pressures and temperatures record values of up to 2.5 kbars and 250°C. These values place limits on the range of thermal conditions in which the hydrocarbon/ 40Ar rad relationship was established. All fluids within inclusions also contain radiogenic 4He 40Ar values at predicted crustal production ratios. These observations provide the first evidence that both 4He rad and 40Ar rad can be quantitatively released on a regional scale bounded by the thermal conditions required to produce the hydrocarbon phase and the conditions under which the fluid inclusions were formed ( T = 190–250° C). These results require that negligible quantities of excess 40Ar rad, decoupled from 4He rad, have been released into this system. Given the wide array of mechanisms which can potentially cause decoupling of these two species, this result provides an important constraint on the role of these processes within the sedimentary fluid regime. In contrast, the free borehole gas contains excess radiogenic 4He and 21Ne, relative to 40Ar rad, in proportions which can be accounted for by local production and subsequent diffusion from the surrounding marl. The latter pattern is consistent with rare gas migration in lower temperature environments. A conceptual model which considers both diffusional and metamorphic release of helium and argon, and the ability of the surrounding fluid regime to transport the rare gases from their respective mineral production sites, is consistent with both these results and data from regional rare gas studies.